diff --git a/assembler.js b/assembler.js
index 58c3f3c..4901daf 100644
--- a/assembler.js
+++ b/assembler.js
@@ -10,11 +10,15 @@ const {
 // 3 = always print
 // 4 = silent
 const DEBUG_LEVEL = 2;
-let DEBUG = false; // Turn debugging on/off -- set by assemble()
+let DEBUG; // Turn debugging on/off -- set by assemble()
 
-exports.assemble = (str, debug = false) => {
+/**
+ * @param {string} assemblyCode
+ * @param {Boolean} [debug = false]
+ **/
+exports.assemble = (assemblyCode, debug = false) => {
   DEBUG = debug;
-  return decodeInstructions(str);
+  return decodeInstructions(assemblyCode);
 }
 
 // Configure pseudo-ops:
@@ -157,7 +161,7 @@ function handleConstantDefinitions(op, arg, IP, constants) {
  * it will be assembled to the default intial value of the IP,
  * as specified in `machine.config.js`.
  * @param {string} source - Assembly source to decode
- * @return TODO
+ * @return {{ debugInfo: Object, machineCode: Uint8Array }};
  **/
 function decodeInstructions(source) {
   dbg(1, 'Pre-parsing...');
@@ -183,8 +187,10 @@ function decodeInstructions(source) {
 
   // Initialize arrays to collect assembled code
 
-  /** @type {array} - Assembled source code, as an array of bytes **/
+  /** @type {Array<number>} - Assembled source code, as an array of bytes **/
   let machineCode = new Array(IP).fill(0);
+  
+  let debugInfo = {};
 
   // Initialize memory-mapped IO -- TODO this should probably be in the CPU, not here
   machineCode[POINTER_TO_DISPLAY] = DISPLAY_ADDR;
@@ -194,6 +200,7 @@ function decodeInstructions(source) {
   let labels = {};
   let constants = {};
 
+
   // Decode line by line...
   for (let i = 0; i < lines.length; i++) {
     let line = lines[i];
@@ -311,6 +318,14 @@ function decodeInstructions(source) {
       machineCode[IP] = decodedOp;
       machineCode[IP + 1] = decodedArg;
 
+    debugInfo[IP] = {
+      lineNumber: line.number,
+      source: line.source,
+      address: IP,
+      machine: [decodedOp, decodedArg]
+    };
+
+
       dbg(3, ``);
       dbg(3, `Line ${line.number}: ${line.source}`);
       if (line.argument) {
@@ -341,7 +356,7 @@ function decodeInstructions(source) {
     }
   }
 
-  return new Uint8Array(machineCode);
+  return { 'debugInfo': debugInfo, 'machineCode': new Uint8Array(machineCode) };
 }
 
 
diff --git a/cpu.js b/cpu.js
index d79f5f8..f71bb6a 100644
--- a/cpu.js
+++ b/cpu.js
@@ -313,7 +313,7 @@ function startCPU(code) {
 /**
  * Execute just the next instruction in memory
  **/
-async function stepCPU(debug = false) {
+async function stepCPU(debugInfo, debug = false) {
   if (CYCLE_LIMIT) { // Temporary limit as a lazy way to halt infinite loops
     if (CPU.cycleCounter > CYCLE_LIMIT) {
       console.warn('HALTING - reached cycle limit');
@@ -332,31 +332,33 @@ async function stepCPU(debug = false) {
       CPU.cycleCounter += 1;
     }
   }
-  logCPUState(debug);
+  logCPUState(debugInfo, debug);
 }
 
 /**
  * @param {Uint8Array} code        - Machine code to run
+ * @param {any}        debugInfo   TODO type
  * @param {Boolean}   [debug]      - Enable/disable debugging printouts
  * @param {Number}    [clockSpeed] - CPU clock speed in milliseconds
  **/
-exports.runProgram = (code, debug = false, clockSpeed = 100) => {
+exports.runProgram = (code, debugInfo, debug = false, clockSpeed = 100) => {
   startCPU(code);
   // Animate the output by pausing between steps
   const loop = setInterval(async () => {
-    stepCPU(debug);
+    stepCPU(debugInfo, debug);
     if (!CPU.running) clearInterval(loop);
   }, clockSpeed);
 }
 
 /**
  * @param {Uint8Array} code   - Machine code to run
+ * @param {any}        debugInfo - TODO
  * @param {Boolean}   [debug] - Enable/disable debugging printouts
  **/
-exports.singleStepProgram = (code, debug = false) => {
+exports.singleStepProgram = (code, debugInfo, debug = false) => {
   startCPU(code);
   while (CPU.running) {
-    stepCPU(debug);
+    stepCPU(debugInfo, debug);
     // FIXME: this prevents exiting with Ctrl+C:
     let key = readlineSync.keyIn('S to step, Q to quit > ', {
       limit: ['s', 'S', 'q', 'Q'],
@@ -372,8 +374,9 @@ exports.singleStepProgram = (code, debug = false) => {
 /**
  * @param {Boolean} [debug] - Enable/disable debugging printouts
  **/
-function logCPUState(debug = false) {
-  console.group(`Step`);
+function logCPUState(debugInfo, debug = false) {
+  console.group(`Step ${CPU.cycleCounter}`);
+    console.log();
     if (!debug) console.clear();
     if (debug) {
       display.printDisplay(CPU.memory);
@@ -381,11 +384,13 @@ function logCPUState(debug = false) {
       display.prettyPrintDisplay(CPU.memory);
     }
     console.log();
+    console.log(`Line ${debugInfo[CPU.IP-2].lineNumber}: ${debugInfo[CPU.IP-2].source}`);
     console.log('Mnemonic:', CPU.currentInstruction.mnemonic);
     console.log(`Machine:  $${num2hex(CPU.currentInstruction.opcode)} $${num2hex(CPU.currentInstruction.operand)}`);
     console.log();
     console.log(`IP: $${num2hex(CPU.IP)}  Acc: $${num2hex(CPU.Acc)}  ONZC: ${num2bin_4bit(CPU.FLAGS)}`);
     console.log(`KEY: $${num2hex(CPU.memory[KEYPAD_ADDR])}  ${CPU.running ? "running" : "halted" }`);
     console.log();
+    console.log();
   console.groupEnd();
 };
\ No newline at end of file
diff --git a/logging.js b/logging.js
index 9922e61..2e1d581 100644
--- a/logging.js
+++ b/logging.js
@@ -6,18 +6,17 @@
  * @param {number}  [end] - An end-index, in decimal
  **/
 const logMemory = (mem, start=0, end=mem.length) => {
-  // This function can only handle
-  // an even number of array entries
-  if ((start % 2) === 1) { start -= 1; }
-  if ((end % 2) === 1) { end += 1; }
-
   mem = mem.slice(start, end);
   console.log(`┌────────┬────────┬─────────┐`);
   console.log(`│  addr  │ opcode │ operand │`);
   console.log(`├────────┼────────┼─────────┤`);
-  //for (let i = 0; i < mem.length; i += 2) { 
   for (let i = start; i < mem.length; i +=2) {
-    console.log(`│   ${num2hex(i)}   │   ${num2hex(mem[i])}   │   ${num2hex(mem[i+1])}    │`);
+    let operand = mem[i+1];
+    if (typeof operand === 'undefined') {
+      console.warn('     operand undefined');
+      operand = -1;
+    }
+    console.log(`│   ${num2hex(i)}   │   ${num2hex(mem[i])}   │   ${num2hex(operand)}    │`);
 
     // Add a blank row every 4 lines:
     if (((i + 2) % 8) === 0) {
diff --git a/notes/issues/todo.md b/notes/issues/todo.md
index 4de1ae3..59ecb67 100644
--- a/notes/issues/todo.md
+++ b/notes/issues/todo.md
@@ -9,7 +9,6 @@ For extended commentary, see [issues](issues.md).
 ### Todo
 
 - Add a flag for bank-switching to the ~zero-page
-- (assembler) Pass asm line thru to cpu to print when debugging
 - Remove run-scripts and add the ability to run `./cpu.js` and `./assembler.js` directly -- cf. [#1](issues.md#1---improve-cli-interface)
 - [fix] (cpu) Make single-stepping work with simulated keypad
 
@@ -41,4 +40,6 @@ For extended commentary, see [issues](issues.md).
 - Minimal LOGO-ish interpreter for turtle graphics
 
 
-## Closed
\ No newline at end of file
+## Closed
+
+- (assembler) Pass asm line thru to cpu to print when debugging
\ No newline at end of file
diff --git a/run-assembler.js b/run-assembler.js
index e8671ac..12a22f4 100755
--- a/run-assembler.js
+++ b/run-assembler.js
@@ -13,21 +13,21 @@ const mode = process.argv[2];
 const filename = process.argv[3];
 const inputFile_str = fs.readFileSync(filename, 'utf8');
 
-let machineCode;
+let assembler_output;
 
 if (mode === "debug") {
-  machineCode = assembler.assemble(inputFile_str, true);
+  assembler_output = assembler.assemble(inputFile_str, true);
   console.log('');
   console.group("Machine code output");
-  logMemory(machineCode);
+  logMemory(assembler_output.machineCode);
   console.groupEnd();
 } else {
-  machineCode = assembler.assemble(inputFile_str);
+  assembler_output = assembler.assemble(inputFile_str);
   let output = '';
   if (mode === 'runbin') { // print binary output
-    machineCode.forEach((n) => output = `${output} ${num2bin(n)}`);
+    assembler_output.machineCode.forEach((n) => output = `${output} ${num2bin(n)}`);
   } else { // print hex output
-    machineCode.forEach((n) => output = `${output} ${num2hex(n)}`);
+    assembler_output.machineCode.forEach((n) => output = `${output} ${num2hex(n)}`);
   }
   console.log(output);
 }
\ No newline at end of file
diff --git a/run-cpu.js b/run-cpu.js
index ec93b14..eb07704 100755
--- a/run-cpu.js
+++ b/run-cpu.js
@@ -16,15 +16,15 @@ const mode = process.argv[2];
 const filename = process.argv[3];
 const inputFile_str = fs.readFileSync(filename, 'utf8');
 
-let machineCode = assembler.assemble(inputFile_str);
+let assemblerOutput = assembler.assemble(inputFile_str);
 if (mode === "debug") {
   logRunningHeader();
-  computer.runProgram(machineCode, true);
+  computer.runProgram(assemblerOutput.machineCode, assemblerOutput.debugInfo, true);
 } else if (mode === "stepdebug") {
   logRunningHeader();
-  computer.singleStepProgram(machineCode, true);
+  computer.singleStepProgram(assemblerOutput.machineCode, assemblerOutput.debugInfo, true);
 } else if (mode === "step") {
-  computer.singleStepProgram(machineCode, false);
+  computer.singleStepProgram(assemblerOutput.machineCode, assemblerOutput.debugInfo, false);
 } else {
-  computer.runProgram(machineCode, false, 200);
+  computer.runProgram(assemblerOutput.machineCode, assemblerOutput.debugInfo, false, 200);
 }
\ No newline at end of file